This invention relates to a method and apparatus for mitigating or reducing supersonic/shock noise generated by aircraft engines, by creating flow structures that form virtual aerodynamic shapes, e.g., lobes, that reduce shock cell strength and enhance mixing.
In military and some commercial aircraft jet engines, the jet engine exhaust is supersonic and contains shock cell structures that create significant noise due to the interaction between the shock cell structures and the shear in the jet flow. These shock cells are primarily generated during the engine nozzle's off-design operation. Stated differently, significant shock cells are generated when the engine nozzle is in either an under-expanded or over-expanded state, depending on the mismatch between the driving pressure and the ideally expanded pressure ratio that would correspond to the current geometry of the engine nozzle.
Additionally, as mixing is significantly suppressed in supersonic shear flows, traditional means of reducing jet engine noise for those with subsonic exhausts are ineffective unless applied on a large scale, which can result in significant system penalties, such as increase in weight and drag, and loss of thrust and operability.
Previous attempts to address shock cell noise in jet engines have proven to be less than desirable. In some applications the use of variable iris nozzles allow for some limited control on the degree of over and under expansion of the nozzle. However, the requirements on thrust and actuation limits in the area control mechanisms limit their use to reduce shock-noise. Further, in other applications the use of passive devices, such as mechanical lobe mixers, have been explored. However, as these devices are passive and exist in the flowpath at all times, the engine system must absorb the penalties of these devices being in the flowpath at all times, even when not necessary. Even if there are no performance penalties, the immersion in a hot flowpath limits their life/reliability and has a negative impact on system cost.